Digital circuits often comprise or include logic circuits which have a speed of operation based upon their delay time, which in turn varies with the applied power supply voltage. This variation in delay time can be a source of jitter in the logic system. One solution to this jitter problem is the introduction of a regulator which holds the voltage provided to the logic circuit constant, thus lessening the jitter. For example, a regulator may be made to operate from a typical 1.2 volt (V) power supply and generate an 800 millivolt (mV) constant voltage for the critical elements of the logic design, such as the delay elements in a delay line.
A regulator designed for this purpose should have certain characteristics in order to properly maintain a steady voltage. First, the output voltage must be provided even when the input voltage is high or low. A typical specification might call for the regulator to provide the desired output when the input voltage varies by +/−15%. Thus, in the above example with an input voltage of 1.2 V, the input voltage may run from about 1.38 V to 1.02 V, and even at these high and low voltages the regulator should still produce the desired output voltage of 800 mV.
Secondly, to be effective the regulator should have a low output impedance even at high frequencies in the output terminal. If it does have a low output impedance, high frequency disturbances will create noise and introduce errors. Finally, it is desirable that the regulator draw the minimum power possible from the voltage supply so that battery life and excess heat are minimized.
One type of simple and inexpensive regulator used to maintain a steady voltage is a linear regulator. The resistance of the regulator varies in accordance with the load on the output, resulting in a constant output voltage. A voltage divider network uses a transistor or other device as a regulating device which is made to act like a variable resistor. The output voltage is compared to a reference voltage to produce a control signal to the transistor, and the transistor continuously adjusts to maintain a constant output voltage. With negative feedback and good compensation, the output voltage is kept reasonably constant.
All linear regulators require an input voltage that is at least some minimum amount higher than the desired output voltage. That minimum amount of excess voltage is called the dropout voltage. In a case where the difference between the supply voltage and the desired output voltage is small, such as the example above of 1.2 V and 800 mV (and as is common in low-voltage power supplies for digital logic circuits), the regulator must be of what is known as a “Low Dropout voltage” type (LDO).
Linear regulators are often inefficient. Because the regulated voltage of a linear regulator is always lower than input voltage, the input voltage must be high enough to always allow the active device to drop some voltage. Further, since the transistor is acting like a resistor, it will waste electrical energy by converting the difference between the input voltage and the regulated output voltages to waste heat.
Linear regulators exist in two basic forms, series regulators and shunt regulators. In the series regulator, the regulating device is placed between the source and the regulated load. In a shunt regulator, the regulating device is placed in parallel with the load. FIG. 1 shows a prior art series regulator, and FIG. 2 shows a prior art shunt regulator.
Series regulators are the more common form. As can be seen in FIG. 1, the series regulator 100 works by providing a path from the supply voltage DVcc to the load resistance 102 through a variable resistance created by a transistor 104. The output voltage Out is equal to the voltage drop over the load impedance, here shown as resistor 102, and is fed back to op amp 106. Op amp 106 is a differential amplifier and amplifies the difference between Out and a voltage from capacitor 108 (in this example, the desired output voltage of 800 mV), and its output remains stable when its inputs are the same.
The output of op amp 106 is fed to the gate of transistor 104, and controls the current passing through transistor 104. Series regulator 100 is thus a closed loop which operates to maintain an output voltage by controlling the amount of current delivered to the load resistance 102. If the current delivered results in the output voltage being too high, the current is reduced, while if the current delivered results in the output being too low it is increased. By this mechanism, a stable output voltage is obtained. The power lost and dissipated as heat is equal to the power supply output current times the voltage drop in the regulating transistor 104.
By comparison, the shunt regulator 200 of FIG. 2 works by providing a fixed current source 202 along with the supply voltage DVcc. The fixed current flows through two paths rather than one as in the series regulator, one path through the load impedance, again shown as a resistor 204, and a second path through the variable resistance provided by transistor 206. The current through transistor 206 is diverted away from the load resistance 204 and flows to ground; it is this current path around the load resistance 204 that provides the regulation of voltage. Like op amp 106 of series regulator 100 in FIG. 1, op amp 208 is a differential amplifier and similarly amplifies the difference between Out and a voltage from capacitor 210 (in this example again the desired output voltage of 800 mV), and is similarly stable when its inputs are the same.
It may be seen that shunt regulator 200 functions somewhat like a zener diode, i.e., the regulator 200 exhibits an abrupt change in incremental resistance at a distinct voltage, i.e., the regulated voltage or zener voltage. Below this voltage the impedance is high, since the effective impedance of transistor 206 is very high and the combined parallel impedance of transistor 206 and load resistor 204 is close to the impedance of load resistor 204, while above this voltage the impedance is low since the effective impedance of transistor 206 is lower, reducing the combined impedance.
This abrupt change in incremental resistance allows the shunt regulator 200 to provide a stable output voltage for a wide range of load conditions at the same regulated or zener voltage. In addition, compared to a series regulator in which the output impedance increases with frequency, a shunt regulator has a lower output impedance as frequency increases and thus may work better in suppressing jitter. FIG. 3 shows curves of impedance over a frequency range for typical series and shunt regulators. As may be readily seen, the impedance of both regulators is about the same until about 50 kilohertz (KHz) or so. However while the impedance of the shunt regulator is constant to about 100 megahertz (MHz), and even drops above that frequency, the impedance of the series regulator increases significantly over about 100 kilohertz (KHz).
However, this flexibility with respect to load conditions and frequency comes at a price. The shunt regulator 200 only works because it wastes current, i.e., it always sinks more current than the maximum current expected, and will thus drain a battery quickly. For example, as shown shunt regulator 200 shows an 8 kilohm (kΩ) load on the 800 mV output; that 8 kΩ load draws 100 microamps (uA), but the shunt regulator 200 wastes another 100 uA or so in the transistor 206. Because the shunt regulator uses more than the “ideal” current, i.e., only what is necessary to go through the load resistance, the shunt regulator is not as efficient as a series regulator under the same conditions.
A designer is thus faced with a choice between a series regulator, which is more efficient but has high output impedance at high frequency, or a shunt regulator, which generally has an inherently low output impedance even at high frequency but is inefficient.
It would thus be desirable to find a simple solution that would combine the frequency response and load flexibility of a shunt regulator with the lower current, and thus lower power drain and waste heat, of a series regulator, for use with logic circuits and other types of electronic circuitry as well.